I just deleted the printer from both https://www.google.com/cloudprint/#printers and https://www.hpconnected.com/#devices/, then disabled Web Services from the printer.
After this, I set everything up from scratch as follows:
1) re-enable Web Services from printer
2) Add the printer at https://www.hpconnected.com/#devices/
3) Add the printer at https://www.google.com/cloudprint/learn/setup-hp.html
Sadly, the problem remains: I go to a color jpg on the web, print via command-P, choose the newly set up printer as the destination, and print. The result is a black-and-white print, not a color one.
Interestingly, the 'Try it Now' link at https://www.google.com/cloudprint/learn/setup-hp.html# gives me a different dialog. Choosing color here yields a color test page. But printing any other page still yields B&W.